A key function of the bus capacitor is to smooth the bus voltage and provide transient current at the switching moment to keep the ripple of the bus voltage small enough.

When the PWM switching frequency is increased, the requirements for bus capacitance are reduced. Because the switching time of the FET becomes shorter, the amount of charge required by the capacitor becomes smaller, so using a higher PWM switching frequency can reduce the required bus capacitance value.

Usually, electrolytic capacitors are used as bus capacitors. Electrolytic capacitors can provide sufficient capacitance, but also have disadvantages, such as huge size, short life, poor high-frequency characteristics, and so forth. In comparison, ceramic capacitors are more stable and smaller, but the capacitance ceramic capacitors can provide is limited. The following test attempts to replace electrolytic capacitors with ceramic capacitors by increasing the PWM frequency.

Determine whether smaller ceramic capacitors can be used by testing the bus ripple of electrolytic capacitors and ceramic capacitors at different frequencies. This test used 60μF and 100μF ceramic capacitors (PN: C3225X7R2A106K250AC × 6 or 10) and 100μF electrolytic capacitor (PN: ECA2AM101).

Figure 4-18 DC-Link Voltage AC Ripple at 48VDC, 8kHz–80kHz PWM, PH_I = 5A_RMS

As Figure 4-18 shows, as the frequency increases, the ripple on the bus gradually decreases, so capacitors with smaller capacitance can be used. But ceramic capacitors have significantly larger voltage ripple at low frequencies (< 80kHz). Because the actual capacitance of this 10μF ceramic capacitor is only 2.2μF at a voltage of 50V, the actual effective capacitances corresponding to the 60μF and 100μF ceramic capacitors in Figure 4-18 are 13.2μF and 22μF. Therefore, the ripple is larger than that of the 100μF electrolytic capacitor.

When the PWM frequency increases to 80kHz, the voltage ripple of a 100μF ceramic capacitor and an electrolytic capacitor is similar. Therefore, the ultra-low switching loss of GaN can be used to increase the PWM frequency to 80kHz. At the same time, the electrolytic capacitor can be replaced with a ceramic capacitor of the same capacity to achieve a smaller size.

Figure 4-19 Ceramic Capacitor Capacitance vs Voltage Curve